The present invention relates to a biosensor simply enabling rapid and high accuracy quantification of a measuring subject in a sample.
A biosensor has conventionally been proposed in the Japanese Laid-Open Patent Publication Hei 2-062952 as a system for simplified quantification of a specific component in a sample without diluting or agitating a sample solution.
This biosensor is completed by first forming an electrode system comprising a measuring electrode, a counter electrode and a reference electrode on an electrically insulating base plate by using a screen printing method or the like, and then forming an enzyme reaction layer comprising a hydrophilic polymer, an oxidoreductase and an electron mediator. If occasion demands, a buffer is added to this enzyme reaction layer.
Upon dropping a sample solution containing a substrate on the enzyme reaction layer thus formed, dissolution of the enzyme reaction layer takes place, which in turn triggers reaction between the enzyme and the substrate, causing a reduction of the electron mediator. Upon completion of the enzyme reaction, this reduced electron mediator is oxidized electrochemically. The concentration of the substrate in the sample solution can be determined by reading the oxidation current occurring in this procedure.
This biosensor can be used theoretically for measurements of various materials if an appropriate enzyme corresponding to the substrate of a target material is selected. For example, the use of glucose oxidase as the oxidoreductase can yield a biosensor for measurement of blood glucose level. This sensor is widely applied practically as a glucose sensor. The use of cholesterol oxidase as the oxidoreductase can yield a biosensor for measurement of serum cholesterol.
Serum cholesterol level which serves as diagnostic standard at various medical institutions is a sum of serum cholesterol and cholesterol ester concentrations. Since cholesterol ester cannot serve as a substrate for oxidation by cholesterol oxidase, a biosensor in which cholesterol oxidase is contained in a reaction layer cannot measure serum cholesterol level as diagnostic standard.
Therefore, a process for changing cholesterol ester into cholesterol is required. Cholesterol esterase is known as an enzyme for catalyzing this process. Inclusion of this cholesterol oxidase together with cholesterol oxidase in the enzyme reaction layer constitutes a biosensor for measurement of the total cholesterol concentration in serum.
The enzyme reaction layer of the biosensor having such constitution is formed by dropping a mixed aqueous solution containing an oxidoreductase, an electron mediator and the like onto the above-mentioned electrode system and drying the dropped solution. Such a procedure causes a problem that when the amount of reagents is large, the reaction layer is not dissolved quickly in dropping of the sample solution onto the reaction layer, and a long period of time is required for the measurement.
Particularly, in the sensor for measurement of the total cholesterol concentration in serum as described above, two kinds in total of enzymes, cholesterol oxidase and cholesterol esterase have to be contained in the enzyme reaction layer. Therefore, the amount of contained reagents increases significantly, so that a longer period of time is necessary for dissolution of the enzyme reaction layer after dropping of the sample solution, giving no quick measurement.
When the reagents such as an oxidoreductase and an electron mediator are contained in the enzyme reaction layer in such a condition that they are mixed with each other, the reagents may be degraded. Particularly, in the case of a long-period storage at high temperature, problems regarding deterioration of sensor response occur such as observation of large current value even if the substrate for the enzyme reaction is not contained in the sample solution.
In view of the above-described drawbacks, an object of the present invention is to provide a biosensor which enables quick measurement by enhancing dissolution of reagents.
Another object of the present invention is to provide a biosensor which maintains excellent response characteristic even after a long-period storage.
A biosensor in accordance with the present invention comprises an electrically insulating base plate, an electrode system having at least a measuring electrode and a counter electrode formed on the base plate, a cover member which is integrated to the base plate so as to form a sample solution supply path for supplying a sample solution to the electrode system between the cover member and the base plate, and a carrier composed of fiber supporting a reagent containing at least an oxidoreductase, wherein the carrier is placed in the sample solution supply path.
Another biosensor in accordance with the present invention comprises an electrically insulating base plate, an electrode system having at least a measuring electrode and a counter electrode formed on the base plate, and a carrier composed of fiber supporting a reagent containing at least an oxidoreductase, wherein the carrier is fixed in the vicinity of the electrode system by an adhesive.
It is preferable that the above-described carrier is constituted at least of two carrier pieces and each carrier piece supports a different reagent.